Fulminate Crystals

Mercury fulminate crystallizes from water as a yellow coloured product containing H20 (Shishkov [19]). It was believed at first that the yellow colour is due to the presence of mercuric oxide resulting from the hydrolysis of the fulminate. It is now considered that this colour should be ascribed to the formation of products not fully defined and partly to the mercuric salts of metafulminuricacid (p. 134).Furthermore on boiling in water, the hydrolysis of mercury fulminate may be fairly extensive. 

Mercury fulminate crystals are not so hard as those of lead azide (Todd and Parry [28]). 

These authors studied the influence of the various methods of treatment on the thermal decomposition of mercury fulminate crystals. This is shown in the graph (Fig. 31). 

Singh [24] noticed that when heated for a few minutes at a temperature nearing that of immediate decomposition mercury fulminate crystals undergo decomposi-... 

Figure 94. Mercury Fulminate Crystals for Use in Primer Composition...

Mercury fulminate has a specific gravity of 4.45, but a mass of the crystals when merely shaken down has an apparent density (gravimetric density) of about 1.75. In detonators the material is usually compressed to a density of about 2.5, but densities as high as 4.0 have been obtained by vigorous compression. Mercury fulminate crystallizes from water in crystals which contain y.H20, from alcohol in crystals which arc anhydrous. One liter of water at 12° dissolves 0.71 gram, at 49° 1.74 grams, and at 100° 7.7 grams. 

After cooling the charge in the flask, it was dumped onto a cloth screen and the crystals of MF washed with cold running water to remove excess nitric acid and impurities known as "fulminate mud ... 

The Fulminate detonated the Tetryl pellets from which the. detong wave passed thru the TNT crystals to the cast TNT and so on, with increasing velocity to the main charge of Amatol (Ref 1, pp 162—63)... 

Mercury fulminate consists of octahedral crystals, belonging to the orthorhombic system with the axial relationship a b c=0.712 1 1.353 (Miles [13]). The pure substance crystallizes into the form of white, silky needles. 

FlO. 28. Crystals of grey mercury fulminate, according to Kast [17]. 

On the basis of all these experiments Kast [17] expressed the opinion that different colours are produced by differences in the size and shape of the crystals and not by impurities. This however appears to be incorrect. There seems every reason to believe that the grey and particularly the brown colour of mercury fulminate are produced by the presence of organic impurities, i.e. resinous product of the polymerization of fulminic acid (Marshall [18]). The white product, in Marshall s opinion, contains the same impurities, but in disguised form. 

The apparent density of loosely-poured fulminate depends to a great extent upon the size and shape of the crystals. According to various authors it may range from 1.22 to 1.60. A fine crystalline product has a low apparent density, a coarse crystalline product a high one. These variations are of great importance when loading caps with fulminate measured volumetrically. In detonators the density is usually as high as 2.5. 

Pyridine is also a good solvent for mercury fulminate. 14.5 g of mercury fulminate may be dissolved in a 1 g of pyridine on moderate heating. The fulminate may by recovered if the solution is poured into water. Large crystals, an addition compound of mercury fulminate with pyridine, then separate. This compound loses pyridine on drying. 

The difference in the behaviour of the two modifications was due largely to the differences in crystal size. Fine fulminate, ground under water decomposes more rapidly than a coarse crystalline product. 

Fig. 32. Crystals of mercury fulminate (a) before heating, (b) after heating at 160°C for 7 min 36 sec, (c) after heating at 80°C for 96 hr, according to Singh [24].

When crystals of mercury fulminate are heated at lower temperatures the decomposition reaction is localized mainly around lattice defects such as growth marks on the surface of crystals or points where dislocations emerge at the surface (Fig. 32(c)). 

The nuclei formed in crystals of mercury fulminate are yellowish-brown. They probably consist of Langhans mercury pyrofulminate [40]. 

According to Patry [22] fulminate darkens under the influence of irradiation by a mercury arc. After a month test samples become almost black and the mercury content increases from 70.8 to 71.3. As on heating, the crystalline structure remains unchanged but the optical properties of the crystals undergo alteration. 

One of the earliest descriptions of the manufacture of mercury fulminate given by Chevalier [59] is as follows 300 g of mercury are dissolved in 3000 g of cold nitric acid (54% HN03, s.g. 1.34) and the solution is poured into a flask containing 1900 g of 90% alcohol. After few minutes a vigorous reaction begins and crystals are precipitated. On completion of the reaction 238 g and 158 g of alcohol are added in turn. The fulminate is filtered off through a cloth filter and carefully washed free of acid with water. The yield is 118-128 parts of fulminate per 100 parts of mercury, i.e. 83-90% of theoretical. 

The fulminate is precipitated in the form of greyish needles. When the reaction is complete, the reactor is allowed to stand for approximately 30 min while the contents are cooled. 1-2 1. of water are then poured in and the liquid is decanted from above the precipitated crystals. The precipitate is transferred to a cloth filter and washed with distilled water until completely free of acid. The product is then screened on a silk sieve (approximately 100 mesh/cm2) which retains the larger crystals. The smaller crystals are collected for direct use. The large ones are ground under water, passed through the same sieve and added to the previous batch. 125 parts of fulminate are obtainable from 100 parts of mercury, which corresponds to a yield of 88%. 

The product was dried in a drier (as described for mercury fulminate) at temperatures from 65 to 70°C in batches of 1.2 kg at a time it was then sieved as described above. A sieve analysis of the product showed, for example, the following sizes of crystals ... 

Mercury fulminate is prepared by dissolving mercury in nitric acid and then pouring into ethanol. A vigorous reaction takes pace which is accompanied by the evolution of white fumes, then by brownish-red fumes and finally again by white fumes. At the same time crystals of mercury fulminate are formed. The crystals are recovered and washed with water until all of the acid is removed. 

The potassium salt, KCHaN3Oa, crystallizes from 80% ale in brilliant, steel-blue ndls, which dec at 21 ° or expl at 220°(Refs 1 2). Its aq soln treated with aq AgNO, and HNOs yields the very explosive silver fulminate... 

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